When handling volatile liquids such as hydrocarbons including gasoline and kerosene, air-volatile liquid vapor mixtures are readily produced. The venting of such air-vapor mixtures directly into the atmosphere results in significant pollution of the environment and a fire or explosion hazard. Accordingly, existing environmental regulations require the control of such emissions.
As a consequence, a number of processes and apparatus have been developed and utilized to recover volatile liquids from air-volatile liquid vapor mixtures. Generally, the recovered volatile liquids are liquified and recombined with the volatile liquid from which they were vaporized thereby making the recovery process more economical.
The initial vapor recovery systems utilized in the United States in the late 1920's and early 1930's incorporated a process combining compression and condensation. Such systems were originally only utilized on gasoline storage tanks. It wasn't until the 1950's that local air pollution regulations began to be adopted forcing the installation of vapor recovery systems at truck loading terminals. Shortly thereafter, the "clean air" legislation activity of the 1960's, which culminated in the Clean Air Act of 1968, further focused nationwide attention on the gasoline vapor recovery problem. As a result a lean oil/absorption system was developed. This system dominated the marketplace for a short time.
Subsequently, in the late 1960's and early 1970's cryogenic refrigeration systems began gaining market acceptance (note, for example, U.S. Pat. No. 3,266,262 to Moragne). While reliable, cryogenic systems suffer from a number of shortcomings including high horsepower requirements. Further, such systems require relatively rigorous and expensive maintenance to function properly. Mechanical refrigeration systems also have practical limits with respect to the amount of cold that may be delivered, accordingly, the efficiency and capacity of such systems is limited. In contrast, liquid nitrogen cooling systems provide more cooling than is required and are prohibitively expensive to operate for this type of application.
As a result of these shortcomings, alternative technology was sought and adsorption/absorption vapor recovery systems were more recently developed. Such a system is disclosed in a number of U.S. patents including, for example, U.S. Pat. No. 4,276,058 to Dinsmore, the disclosure of which is fully incorporated herein by reference. Such systems utilize a bed of solid adsorbent selected, for example, from silica gel, certain forms of porous mineral such as alumina and magnesia, and most preferably activated carbon or charcoal. These adsorbents have an affinity for volatile hydrocarbon liquids. Thus, as the air-hydrocarbon vapor mixture is passed through the bed, a major portion of the hydrocarbons contained in the mixture is adsorbed on the bed. The resulting residue gas stream comprising substantially hydrocarbon-free air is well within regulated allowable emission levels and is exhausted into the environment.
It should be appreciated that the bed of adsorbent used in these systems is only capable of adsorbing a certain amount of hydrocarbons before reaching capacity and becoming ineffective. Accordingly, the bed must be periodically regenerated to restore the carbon to a level where it will effectively adsorb hydrocarbons again. This regeneration of the adsorbent is a two step process.
The first step requires a reduction in the total pressure by pulling a vacuum on the bed to remove the largest amount of hydrocarbons. The second step is the addition of a purge air stream that passes through the bed. The purge air polishes the bed so as to remove substantially all of the remaining adsorbed hydrocarbons. These hydrocarbons are then pumped to an absorber tower wherein lean oil or other nonvolatile liquid solvent is provided in a countercurrent flow relative to the hydrocarbon rich air-hydrocarbon mixture being pumped from the bed. The liquid solvent condenses and removes the vast majority of the hydrocarbons from that mixture and the residue gas stream from the absorber tower is recycled to a second bed of adsorbent while the first bed completes regeneration.
Generally, the vacuum pump utilized to evacuate the bed of adsorbent is typically a conventional liquid seal vacuum pump, also known as a liquid ring vacuum pump. Advantageously, liquid seal vacuum pumps are capable of producing high vacuums. They are also relatively inexpensive and generally safer to operate than other types of vacuum pumps when recovering flammable vapors.
A liquid seal vacuum pump utilizes a seal liquid that is circulated through the pump. The seal liquid, usually an ethylene glycol solution, may be circulated through a closed circuit or conduit. Usually, the seal liquid is also cooled and thereby has the effect of continuously cooling the pump and the gas or gases flowing through the pump. The resulting cooler operating temperatures advantageously serve to maximize the performance of the vacuum pump.
One shortcoming relating to the utilization of liquid seal vacuum pumps in vapor recovery systems of the type described relates to the fact that the seal liquid may be at least partially soluble or miscible with the volatile liquid vapor which the system is designed to recover. Accordingly, under certain operating conditions, the seal liquid may become so diluted with the recovered volatile liquid vapor that it can no longer properly function to allow the vacuum pump to operate at maximum efficiency and effectiveness while also minimizing component wear. Further, some seal liquid may be carried with the air-volatile liquid vapor mixture through the absorber tower and into one or more of the adsorbent beds. There, the seal liquid may detrimentally affect the adsorption capacity of the bed. In severe cases of such seal liquid contamination, it may even become necessary to shut down the vapor recovery system and replace the adsorbent in the beds.
Recognizing these significant potential adverse side effects of operating a liquid seal vacuum pump in a vapor recovery system, it has long been known to provide a separator for recovering the seal liquid from the air-volatile liquid vapor mixture produced during bed regeneration. In the past, such separators have relied upon gravity and the utilization of a demister pad to separate the liquid seal liquid from the air-volatile liquid vapor mixture evacuated from the reaction vessel undergoing bed regeneration. While such separators are relatively efficient and function well for their intended purpose, they do not approach 100% seal liquid recovery efficiency. Accordingly, some seal liquid still passes through the separator with the air-volatile liquid vapor mixture. Some of this seal liquid then passes through the absorber tower and is delivered to one or more of the adsorbent beds as the vapor recovery system cycles. There the seal liquid is captured by the adsorbent, detrimentally affecting the adsorption capacity of the bed. Over time this has the effect of diluting the seal liquid and thereby adversely affecting the operation of the vacuum pump. Further, the loss in the adsorption capacity of the bed gradually worsens until the performance of the vapor recovery system is noticeably impaired. Thus, a need is clearly identified for a new and improved vapor recovery system wherein there is provided more efficient and effective recovery of seal liquid from the air-volatile liquid vapor mixture evacuated from the bed of adsorbent undergoing regeneration.